"This situation is grave. We are here to protect public safety. We believe these are the same individuals that were responsible for the bombing on Monday at the Boston Marathon," Alben said. "We believe that they're responsible for the death of an MIT police officer and the shooting of an MBTA officer."

"When I started hearing the gunshots and explosions. I realized something was really wrong. Given what had happened at MIT and all the sirens, I felt strongly that this was related to what happened earlier in the week."

"First I heard the gunshots and then I heard the explosion," Martin said. "All of a sudden police swarmed the area, so I came out and they told me to get back in. They were swearing. 'Get the (expletive) inside!' I looked out back and there were 20 officers. It was upsetting to see SWAT and police everywhere."